The present invention relates to a method for producing synthetic quartz glass and to a device for producing synthetic quartz glass, which can be used within the scope of the method according to the present invention. The method according to the present invention and the device according to the present invention are characterized by a special membrane filter.
For producing synthetic quartz glass, SiO2-particles are produced from a silicon-containing starting substance in a CVD-procedure by means of hydrolysis or oxidation and deposit on a moving support. The method can be subdivided into external and internal deposition methods. In the case of external deposition method, the SiO2-particles are applied onto the outside of a rotating support. Examples of pertinent external deposition methods include the so-called OVD method (Outside Vapor Phase Deposition), the VAD method (Vapor Phase Axial Deposition) or the PECVD method (Plasma Enhanced Chemical Vapor Deposition). The best-known example of an internal deposition method is the MCVD method (Modified Chemical Vapor Deposition), in which SiO2-particles are deposited on the inner wall of a tube heated from outside.
If the temperature in the area of the support surface is sufficiently high, the SiO2-particles vitrify directly, which is also known as “direct vitrification”. In contrast, the temperature during the deposition of the SiO2-particles in the so-called “soot method” is so low that a porous SiO2-soot layer is obtained, which is then sintered into transparent quartz glass in a separate process step. Both, direct vitrification and the soot method lead to a dense, transparent, high pure, synthetic quartz glass.
Silicon tetrachloride (SiCl4), is known from the prior art as a silicon-containing production material for the production of synthetic quartz glass. Silicon tetrachloride and other analogous chlorine-containing substances possess sufficient vapor pressures already at moderate temperatures below 100° C., such that any impurities usually remain in the liquid phase and the production of ultrapure soot bodies is made easier.
However, it is known that, during the evaporation of the silicon tetrachloride, partially liquid drops are entrained within the inert vapor stream used and are not completely evaporated until the reaction zone has been reached. Thus, the impurities contained in the liquid phase finally reach the soot-body and thus deteriorate the quality of the quartz glass produced thereof. The impurities are usually metals.
A further disadvantage of the chlorine-containing production materials, such as silicon tetrachloride, is the production of hydrochloric acid when converted into synthetic quartz glass, which causes high costs for exhaust gas scrubbing and disposal. Therefore, in principal, when silicon tetrachloride is used, devices which prevent the entry of moisture are used. This reduces the formation of hydrochloric acid and avoids the formation of silica acid. This procedure is familiar to a person skilled in the art.
In the past, in order to circumvent these requirements, a large number of so-called crystal-free organosilicon compounds have been tested as production materials for quartz glass production. Examples include monosilanes, alkoxysilanes, siloxanes and silazanes. A particularly interesting group of these so-called chlorless organosilicon compounds are the polyalkylsiloxanes (also called briefly as “siloxanes”), which are known, for example, from DE 30 16 010 A1. In particular, the cyclic polyalkylsiloxanes which can be subsumed under the polyalkylsiloxanes are distinguished by a particularly high proportion of silicon per weight proportion, which contributes to the economic efficiency of their use in the production of synthetic quartz glass. Due to the industrial availability in high purity, octamethylcyclotetrasiloxane (OMCTS) is used in particular.
Such polyalkylsiloxane compounds are polymerizable and are typically present in the production material in pure form or as a mixture with other components in liquid form. These compounds can also contain traces of polymerizable silanols. The polyalkylsiloxane compounds can be fed to the consuming unit, such as a deposition burner, in the liquid form and sprayed at the burner outlet or in the flame. Usually, however, the production material is converted into a gaseous or vapor phase by means of an evaporator and fed to the consuming unit as a continuous gas stream via a line system.
Several methods for the production of synthetic quartz glass, based on these so-called chlorine-free production materials, are described in the prior art. For example, reference is made to EP 0 760 373 A, WO 99/15468 A, WO 99/54259 A, WO 2013/092553 A and EP 0 529 189 A.
However, the use of polyalkylsiloxanes involves other difficulties with regard to possible influences of impurities of the production material on the quality of the resulting quartz glass, in a fundamental difference from the methods described above, in which the low molecular metallic impurities in the silicon tetrachloride reduce the quality of the resulting quartz glass and at the same time, the entry of moisture is avoided. In principle, silicon tetrachloride is thermally more stable than the cyclic polyalklysiloxanes and their boiling point is much lower. On the other hand, the cyclic polyalkylsiloxanes do practically react with moisture at room temperature. However, one of the main problems associated with the use of cyclic polyalkylsiloxanes is that polymerizations take place with formation of gel-like and rubbery residues under the used evaporation conditions. As already stated above, it is known that polyalkylsiloxanes may contain traces of polar impurities such as water, silanols and sometimes even polymerization-catalytically-acting trace components (e.g. Lewis acids or Lewis bases). In the case of the silanols, these impurities can either react with themselves to form polymers or initiate ring-opening reactions with the starting compound. This ultimately leads to the formation of the abovementioned polymer siloxane residues and gels. These polymers and gels usually remain in the evaporator, in the steam lines, control valves, throttles, other gas metering devices and lines, and are concentrated therein. This can lead to a massive impairment of the control behavior of the material streams. Reproducible process management is thus made difficult. In extreme cases, this leads to clogging. Both effects increase the downtime for maintenance and cleaning steps, whereby a process using polyalkylsiloxanes entails pertinent costs.
On the other hand, these residues also have a negative effect on the properties of the resulting quartz glass, since the equal distribution of the mass flows of the production material vapor is uncontrollable and thus not reproducibly affected. This increases the radial and axial density variation in the soot and the variation in the chlorine content in the subsequent dehydration or chlorination step. Furthermore, such residues lead to an increase in the outside diameter variation in a multi-burner method. This, in turn, has an impact on the rejects associated with corresponding material loss. This results in poorer process efficiency combined with increased production costs. In addition, the production material is generally subject to certain batch variations, which are in the ppm range, but nevertheless contribute to the above-described lack of controllability and reproducibility of the process. Raw materials from different manufacturers also have different impurities/contamination levels, so that the control over the quality of the resulting quartz glass is not ensured.
A poorer process efficiency combined with increased production costs is the consequence. In addition, the starting material is, in itself, generally subject to certain batch fluctuations which, although they are in the ppm range, nevertheless contribute to the above-described controllability and reproducibility of the process. As further prior art which relates to the problem of gel formation during the vaporization of polyalkylsiloxanes, mention may be made of U.S. Pat. No. 5,879,649, EP 1 094 990 A, WO 2013/092553 A, EP 0 463 045 A, U.S. Pat. No. 5,970,751, and US Patent Application Publication No. 2012/0276291 A.
To solve these problems, U.S. Pat. No. 5,558,687 proposes to spray the polyalkylsiloxane component initially and to apply it partially in liquid form to a packing material. A similar procedure is described in EP 0 765 845 A in connection with the use of silicon tetrachloride.
The solution possibilities disclosed in the prior art are still not yet satisfactory. An objective of the present invention is therefore to provide a method in which the gel formation in the production of synthetic quartz glass is reduced.
An objective of the present invention is also to provide a method and an apparatus for producing synthetic quartz glass, in which the mass flows of the production material vapor are essentially controllable and thus reproducible.
A further objective of the present invention is to provide a method and a device for producing synthetic quartz glass in which, in particular, the radial and axial density variation in the soot and the variation in the chlorine content are reduced in the subsequent dehydration or chlorination step.
A further objective of the present invention is to provide a method and a device for producing synthetic quartz glass in which, in particular, the outside diameter variation in a multi-burner process is reduced.
These objectives are achieved by the method according to the present invention for producing synthetic quartz glass, as described below, and the device according to the present invention which is used in the method and which is also described below.